High efficiency foam compacts for triso fuels

ABSTRACT

A nuclear fuel element is provided. The nuclear fuel element includes a porous support. The porous support includes a ligament and defines a pore adjacent to the ligament. The ligament has an interior surface spaced from the pore. The interior surface defines a void. The porous support includes silicon carbide. The nuclear fuel element includes a nuclear fuel material disposed in the pore. The nuclear fuel material includes a moderator and tri-structural isotropic (TRISO) particles. Another nuclear fuel element is provided. The nuclear fuel element includes a porous support. The porous support includes a ligament and defines a pore adjacent to the ligament. The ligament has an interior surface spaced from the pore. The interior surface defines a void. The ligament includes the nuclear fuel material. The nuclear fuel element includes a facesheet overlying the porous support and defines a hole. The hole is in fluid communication with the void. The nuclear fuel material includes a nuclear fuel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application62/908,779, filed Oct. 1, 2019, the disclosure of which is incorporatedby reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No.DE-AC05-000R22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE DISCLOSURE

The present invention relates generally to porous nuclear fuel elements,in particular, for use in advanced high temperature gas-cooled nuclearreactors (HTGRs) and gas-cooled fast reactors (GFRs), and to methods forfabricating same.

BACKGROUND OF THE DISCLOSURE

High temperature gas-cooled nuclear reactors (HTGRs) have been operatedusing pebble-bed core designs with spherical fuel elements (60 mmdiameter “pebbles”) made of TRISO fuel particles embedded in a graphitematrix. Each TRISO fuel particle is a microsphere (0.9 mm diameter)comprising a kernel/core of fissile material (UO₂ or UC₂) coated bymultiple layers of protective barrier materials (also commonly referredto as “cladding”), e.g., a porous carbon buffer layer, followed bypyrocarbon, followed by silicon carbide, followed by an external coatingof pyrocarbon. A typical reactor core might contain 11,000 fuel pebbleseach containing thousands of TRISO particles. TRISO filet is a keycomponent of advanced small modular nuclear reactors due to its inherentsafety at high temperatures and irradiation levels and decreasedproliferation risk relative to current reactors. For HTGRs, a moreefficient cooling method for TRISO fuel elements may increase thermalcontrol and allow for an increase in fuel density.

Gas-cooled fast reactors (GFRs) are high-temperature helium-cooledfast-spectrum reactors with a closed fuel cycle. These features providefor efficient conversion of fertile uranium and management of actinides.The core of GFRs has a high fissile fuel content as well as anon-fissile, fertile, breeding component. There is no neutron moderatorin (IFRs, as the chain reaction is sustained by fast neutrons. Activeventing of fission product gases and the resultant decrease in claddingstress in TRISO fuel elements for GFRs would significantly increase fuellifetime and enable operating with a deep burn fuel cycle.

Highly porous (e.g., 90% porous) metal carbide foam structures have beenfabricated through chemical vapor deposition of one or more layers of arefractory metal carbide, for example, ZrC or NbC, on a porous foamskeleton made of, for example, reticulated vitreous carbon (RVC). Thesemetal carbide foams have been used as thermal protection systems,actively cooled structures/heat exchangers, flash and blast suppressors,and lightweight mirror substrates. The interconnected open cell geometryand tortuous flow path provides excellent heat exchange properties,excellent particulate filtration, with a correspondingly low mass.However, conventional incorporation of nuclear fuel into these highlyporous metal carbide foam structures has not resulted in the coolingnecessary for HTGRs and the gas mobility for GFRs.

Accordingly, there remains a need for improved nuclear fuel elementsexhibiting improved cooling and gas mobility.

SUMMARY OF THE DISCLOSURE

in one embodiment, a nuclear fuel element is provided. The nuclear fuelelement includes a porous support. The porous support includes aligament and defines a pore adjacent to the ligament. The ligament hasan interior surface spaced from the pore. The interior surface defines avoid. The porous support includes silicon carbide. The nuclear fuelelement includes a nuclear fuel material disposed in the pore. Thenuclear fuel material includes a moderator and tri-structural isotropic(TRISO) particles.

In another embodiment, a nuclear fuel element is provided. The nuclearfuel element includes a porous support. The porous support includes aligament and defines a pore adjacent to the ligament. The ligament hasan interior surface spaced from the pore. The interior surface defines avoid. The ligament comprises, consists essentially of, consists of, orif formed from the nuclear fuel material. The nuclear fuel elementincludes a facesheet overlying the porous support and defines a hole.The hole is in fluid communication with the void. The nuclear fuelmaterial includes a nuclear fuel, in various embodiments, the nuclearfuel is further defined as a fissile nuclear fuel.

For high temperature gas-cooled reactors (HTGR), the potential exists tocast and sinter a slurry of TRISO particles and graphite into the opencells of highly porous silicon carbide foam that has hollow ligaments.The foam provides structural reinforcement for the TRISO/graphitemixture and the hollow ligaments, which can be varied in size andspacing, serving as network cooling passages thereby allowing forcoolant flow in much closer proximity to the fuel particles compared tocurrent fuel elements. In addition, by using SiC foam, the thermalconductivity of the fuel element increases. To this end, theinterconnected microchannels allow helium coolant to flow close to thefuel particles for efficient heat removal. The microchannel size andspacing can be varied substantially for the TRISO particles in foamstructure to establish the optimal combination of pressure drop, coolingefficiency, and fuel density.

This design capitalizes on the established technology base regarding therobustness of TRISO fuel encapsulation, and drastically reduces thethermal gradients in large compacts, yet be easy to manufacture. It alsopermits a substantial increase in fuel density enabling lower enrichmentscenarios for the fuel. The development focuses on a thermal reactorneutron spectrum, relying on the graphite in the matrix to provide themoderation. The design also provides the significant thermal massrequired for loss of pressure excursions.

For gas-cooled fast reactors (GFR) using a deep-burn fuel cycle, thepotential exists to make TRISO-like hollow ligament foam in which theligaments are composed of UCO fuel as the inner layer deposited by CVD,followed by the same cladding layers currently deposited over a UCOkernel for TRISO particles.

TRISO fuel is a key component of advanced, small modular nuclearreactors due to its inherent safety at high temperatures and irradiationlevels, and decreased proliferation risk relative to current reactors.However, a more efficient cooling method for the fuel elements mayincrease thermal control and allow for an increase in fuel density.Also, reducing fission product gas buildup and cladding stress wouldsignificantly increase fuel lifetime for both fast and thermal reactors.For the proposed TRISO/graphite infiltrated foam for HTGR, the foamprovides structural reinforcement for the TRISO/graphite mixture and thehollow s serve as a network cooling passages, thereby allowing forcoolant flow in much closer proximity to the fuel particles compared tocurrent fuel elements and potentially allow for an increase in fueldensity.

For the proposed vented TRISO foam fuel for GFR using a deep-burn fuelcycle, the hollow ligaments are used to vent fission products. If notremoved, the fission gases stress the fuel matrix and cladding, limitingthe lifetime of the fuel and its burn-up level. Fission product gasescan be removed from the fuel and transported to a reflector or blanketfor more efficient transmutation, or completely removed from the core toan on-site facility where it can be processed for long-term storage. Thecontrolled removal of fission gases from the fuel is critical toachieving deep bum-up and realization of the benefits which includesignificantly reducing the volume and toxicity of nuclear waste,minimizing proliferation risk, and increased power efficiency byutilizing more of the energy content in the low enriched uranium fuel.

The proposed technology development represents both near-term (HTGR) andlonger term (GFR) applications for hollow ligament foam in small modularreactors, and has potential to significantly improve the performance andmanufacturability of TRISO-based fuel.

These and other features and advantages of the present invention willbecome apparent from the following description of the invention, whenviewed in accordance with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part ofthe specification, illustrate various examples of the present inventionand, together with the detailed description, serve to explain theprinciples of the invention.

FIG. 1 illustrates a schematic view of a non-limiting embodiment of anuclear fuel element.

FIG. 2 is a SEM micrograph magnified 20× of a porous support includingligaments and pores.

FIG. 3 is a SEM micrograph magnified at 17× of a porous supportincluding ligaments and pores.

FIG. 4 is a tomography scan illustrating a non-limiting embodiment of anetwork of interconnected internal microchannels with a SEM micrographoverlaid thereon.

FIG. 5 illustrates a schematic view of a non-limiting embodiment of anuclear fuel clement including a nuclear fuel material.

FIG. 6 illustrates a schematic view of another non-limiting embodimentof a nuclear fuel element including a nuclear fuel material.

FIG. 7A is an image of a side view of a non-limiting embodiment of anuclear fuel element including a nuclear fuel material.

FIG. 7B is an image of a top view of the non-limiting embodiment of thenuclear fuel element of FIG. 7A.

FIG. 8A is an image of a cross-section of another non-limitingembodiment of a nuclear fuel element including a nuclear fuel material.

FIG. 8B is an enhanced image of the cross-section of the non-limitingembodiment of the nuclear fuel element of FIG. 8A.

FIG. 9A is an image of a side view of another non-limiting embodiment ofa nuclear fuel element including a nuclear fuel material.

FIG. 9B is an enhanced image of the side view of the non-limitingembodiment of the nuclear fuel element of FIG. 9A.

FIG. 9C is an image of a top view of the non-limiting embodiment of thenuclear fuel element of FIG. 9A.

FIG. 9D is an enhanced image of the top view of the non-limitingembodiment of the nuclear fuel element of FIG. 9C.

FIG. 10 is a tomography scan illustrating a non-limiting embodiment of anuclear fuel element including a nuclear fuel material.

FIG. 11 is a tomography scan illustrating another non-limitingembodiment of a nuclear fuel element including a nuclear fuel material.

DETAILED DESCRIPTION

FIG. 1 illustrates a schematic view of a non-limiting embodiment of anuclear fuel element 10. The nuclear fuel element 10, and a method formaking the same, are provided. The nuclear fuel element 10 includes aporous support 12. The nuclear fuel element 10 further includes aligament 14. The porous support 12 defines a pore 16 adjacent to theligament 14. In various embodiments, the porous support 12 defines aplurality of pores 16 with the pores in fluid communication with eachother. The ligament 14 has an interior surface 18 spaced from the pore16. The interior surface 18 defines a void 20 within the ligament 14 andextending at least partially through the ligament 14. In variousembodiments, the nuclear fuel element 10 includes a plurality ofligaments 14 with the voids 20 of the ligaments 14 in fluidcommunication with each other. The ligament 14 may be further defined asa hollow ligament defining the void 20 extending from a first opening toa second opening.

The porous support 12 may have a total porosity greater than about 70%.Alternatively, the total porosity may be greater than about 70% and lessthan about 90%. Alternatively, the total porosity may be greater thanabout 77% and less than about 85%. One example of an optimum totalporosity is about 77%, which provides a good balance between heattransfer and pressure drop. This optimum design provides just enoughfissile material to be critical, without resorting to excessively largereactor sizes or using weapons grade enrichments. The fuel matrix isadaptable to both thermal and fast reactors by inclusion or deletion ofmoderator material (e.g. ZrH or C). Having a thin thickness of thenuclear fuel allows for a high total porosity to maintain the heattransfer efficiency and to keep the temperature difference between theligament (fuel) centerline and the coolant (e.g., helium or hydrogen)bulk temperature as low as possible.

FIG. 2 is a SEM micrograph magnified at 20× of the porous support 12including a foam ligament structure before being infiltrated withnuclear fuel. FIG. 3 is a SEM micrograph magnified at 17× of the poroussupport 12 including a foam ligament structure before being infiltratedwith nuclear fuel. FIG. 4 is a tomography scan illustrating a network ofinterconnected internal microchannels with a SEM micrograph overlaidthereon.

The porous support 12 may be formed utilizing any method known in theart so long as the porous support 12 remains suitable for use in anuclear fuel element 10. One exemplary process for forming the poroussupport 12 includes utilizing a reticulated vitreous carbon (RVC) foamskeleton. First, commercially available polyurethane foam may bepurchased in the desired pore size. Then, the polyurethane foam may beinfiltrated with a carbon-bearing resin and pyrolyzed to form a porous,open-celled material comprised of vitreous (glassy) carbon, which iscalled the RVC foam skeleton. Some shrinkage may occur at this step, butthe original pore structure of the polyurethane foam may be retainedduring conversion to RVC. The RVC foam may have an extremely high voidvolume (e.g., 97%), combined with self-supporting rigidity. Poredensities from 3 to 100 pores per linear inch (ppi) are readilyavailable, and higher ppi foams can be made by compressing 100-ppimaterial prior to pyrolysis in one, two, or three dimensions.Compression or stretching can also be used to create directionalproperties (e.g., strength, pressure drop, etc.). Additionally, the RVCfoam skeleton can be machined to near final dimensions prior to vaporinfiltration.

High ppi (e.g., hundreds of ppi) compressed carbon foam may be used asthe skeletal structure for increasing the surface area and heat transferof the nuclear fuel. 65 ppi foam was selected for initial development,but foams up to 130 ppi are readily produced through resin infiltrationof pyrolysis of polyurethane foam, yield vitreous carbon. By compressionof foam prior to the conversion to carbon, foams in excess of 1000 ppihave been fabricated. Compressed foams may be anisotropic structureswith directional fluid flow, thermal, and mechanical properties, whichmay be tailored.

In various embodiments, the porous support 12 (e.g., the ligaments 14)includes one or more materials selected from the group consisting ofcarbon, graphite, SiC, Nb, Zr, Nb, Mo, Hf, Ta, W, Re, TiC, TaC, ZrC,SiC, HfC, BeC₂, B₄C, NbC, GdC, HfB₂, ZrB₂, Si₃N₄, TiO₂, BeO, SiO₂, ZrO₂,Y₂O₃, Al₂O₃, Sc₂O₃, and Ta₂O₅. In one embodiment, the porous support 12(e.g., the ligaments 14) includes silicon carbide (SiC). In anotherembodiment, the porous support 12 (e.g., the ligaments 14) includesniobium (Nb). Alternatively, the porous support 12 may include anopen-celled foam structure/skeleton including a carbon-bearing materialselected from the group of carbon bonded carbon fiber (CBCF) foam,reticulated vitreous carbon (RVC) foam, pitch derived carbon foam(PDCF), and graphite foam.

FIGS. 5 and 6 illustrate schematic views of a non-limiting embodimentsof the nuclear fuel element 10 including a nuclear fuel material 22. Thevoid 20 of the ligament 14 and the pore 16 of the porous support 12cooperate to provide improved cooling and gas mobility to the nuclearfuel element 10. In certain embodiments, the void 20 is substantiallyfree of the nuclear fuel. The phrase “substantially free” as utilizedherein with reference to the void 20 means that the void 20 includes thenuclear fuel in an amount of no greater than 10, no greater than 5, nogreater than 1, or no greater than 0.1 vol %, based on a total volume ofthe void 20. In various embodiments, the pore 16 has a volume and thepore 16 includes the nuclear fuel material in an amount of at least 50,at least 60, at least 70, at least 80, at least 90, or at least 99 vol%, based on the volume of the pore 16. In other embodiments, the pore 16is substantially free of the nuclear fuel. The phrase “substantiallyfree” as utilized herein with reference to the pore 16 means that thepore 16 includes the nuclear fuel in an amount of no greater than 10, nogreater than 5, no greater than 1, or no greater than 0.1 vol %, basedon a total volume of the pore 16.

With particular reference to FIG. 5, in some embodiments, the nuclearfuel material 22 is disposed in the pore 16. In these and otherembodiments, the nuclear fuel material 22 includes a moderator and atri-structural isotropic (TRISO) particle. It is to be appreciated thatthe nuclear fuel material 22 may include a moderator and a plurality ofthe TRISO particles. In certain embodiments, the moderator may includegraphite. In these and other embodiments, the nuclear fuel materialfurther comprises zirconia. For high temperature gas-cooled reactors(HTGR), a slurry of TRISO particles and graphite may be casted andsintered into the pores 16. The porous support 12 may provide structuralreinforcement for the TRISO/graphite mixture and the hollow ligaments,which can be varied in size and spacing, serving as network coolingpassages thereby allowing for coolant flow in much closer proximity tothe fuel particles compared to current fuel elements. In addition, byusing SiC foam the thermal conductivity of the fuel element increases.In various embodiments, the voids 20 permit the flow of helium gas topurge tritium from the nuclear fuel element 10.

With particular reference to FIG. 6, in other embodiments, the ligament14 of the porous support 12 includes, consists essentially of, consistsof, or is formed from, the nuclear fuel material 22. In these and otherembodiments, the nuclear fuel material 22 includes a nuclear fuelincluding atoms of uranium, carbon, and oxygen. For example, the nuclearfuel may include uranium dioxide (UO₂) and uranium acetylide (UC₂) (“UCOfuel”). This TRISO-like porous support 12 in which the ligaments 14 arecomposed of UCO fuel as the inner layer deposited by CVD may include thesame cladding currently deposited over a UCO kernel for TRISO particles.In certain embodiments, the ligament 14 is formed from the nuclear fuel.

The nuclear fuel material 22 may further include a cladding overlyingthe nuclear fuel. The cladding may include a porous carbon buffer layer,followed by pyrocarbon, followed by silicon carbide, and. followed. byan external coating of pyrocarbon. The cladding may protect theunderlying nuclear fuel material 22 from exposure to hot hydrogen gases,and can serve as a barrier to prevent migration of fission products,especially fission product gases. In other embodiments, the cladding mayinclude one or more materials selected. from the group of NbC, ZrC, BeO,BeC₂, ZrC₂, SiC, pyrolytic carbon, diamond, and diamond-like carbon. Thecladding may have any thickness suitable to function as a barriercoating (about 25 microns).

With continuing reference to FIG. 6, the nuclear fuel element 10 isadapted to direct a first fluid 24 through the void 20 and a secondfluid 26 through the pore 16. It is to be appreciated that the firstfluid 24 and the second fluid 26 may be the same or different. The firstfluid 24 or the second fluid 26 may include helium gas. For gas-cooledfast reactors (GFR) using a deep-burn fuel cycle, the first fluid 24 maypurge fission product plenum through the pore 16 and the second. fluid26 may provide cooling to the ligament 14.

The nuclear fuel element 10 includes a facesheet 28 overlying the poroussupport 12. The facesheet 28 defines a hole 30. The hole 30 is in fluidcommunication with the void 20. In certain embodiments, the facesheet 28defines a plurality of the holes 30 and the holes 30 are in fluidcommunication with the voids 20. The facesheet 28 may include, consistessentially of, consist of, or be formed from, niobium.

In some embodiments, the porous support 12 may have a first face 30A, asecond face 30B opposite the first face 30A, a third face 30C transverseto the first face 30A, and a fourth face 30D opposite the third face30C. In various embodiments, a first facesheet 28A overlies the firstface 30A and a second facesheet 28B overlies the second face 30B. Inthis and other embodiments, the ligaments 14 adjacent the third face 30Cand the fourth face 30D are substantially free of voids 20. The phrase“substantially free” as utilized herein with reference to the outerdiameter face 30E means that the third and fourth faces 30C, 30Dincludes the voids 20 in an amount of no greater than 5, no greater than1, no greater than 0.1, or no greater than 0.01%, based on a totalsurface area of the third and fourth faces 30C, 30D. By controlling flowof the first fluid 24 and the second fluid 26, the nuclear fuel element10 exhibits a balance between (1) maximizing fuel volume to keepenrichment acceptable (50+ % vol % dense foam fuel possible), (2)maximizing open volume outside ligaments 14 for optimal helium cooling,and (3) minimizing open volume inside hollow ligament 14 purge channelsfor optimal fission gas removal.

Exemplary nuclear fuel elements 10 will now be described with referenceto FIGS. 7-11. It is to be appreciated that the exemplary nuclear fuelelements 10 are non-limiting and may be in any configuration known inthe art.

FIGS. 7A and 7B are images of side and top views of a non-limitingembodiment of the nuclear fuel element 10 including the nuclear fuelmaterial 22. A slurry of 650-μm surrogate TRISO particles and graphitepaste may be cast into the pores 16 of a 90% porous SiC foam, followedby an elevated temperature sintering and a surface machining to exposethe void 20 of the ligaments 14. As a result, the nuclear fuel element10 may be formed including surrogate TRISO particles embedded ingraphite reinforced with SiC foam containing an interconnected networkof cooling channels (i.e., the voids 20 in fluid communication with eachother). The particle concentration may be from at least 20% vol. %,alternatively at least 30 vol. %, or alternatively at least 45 vol. %,based on a total volume of the pores 16. The size, spacing, and wallthickness of the ligaments 14 can be varied substantially to achieve theoptimal combination of coolant flowrate, pressure drop, and fuel volume.The TRISO matrix may be infiltrated into foam that has the voids 20already present of the faces 30 of the porous support 12 withoutmachining. To prevent particles from entering the voids 20 during foaminfiltration, wax can be used to temporarily block the passages and itcan later be removed through high temperature exposure to re-open thepassages.

FIGS. 8A and 8B are images of a cross-section of another non-limitingembodiment of the nuclear fuel element 10 including the nuclear fuelmaterial 22. In these and other embodiments, the pores 16 between theligaments 14 (“hollow foam ligaments”) may be vacuum-infiltrated with aTRISO surrogate/graphite slurry, followed by high temperature pyrolysis.Pyrolytic carbon (PyC)-coated ZrO₂ particles (800 μm dia.) may be usedas the TRISO surrogate. The hollow ligament foam structure is clearlyevident in FIGS. 7A and 7B. It is to be appreciated that sectioning ofthe nuclear fuel element 10 shown in in FIGS. 7A and 7B resulted inpartial removal of the ZrO₂ particles of the TRISO surrogate from thePyC shell at the surface of the cross-section. Any remaining unfilledpores 16 may be filled with the TRISO surrogate utilizing anoptimization of the vacuum infiltration process. In certain embodiments,an injection molding at elevated pressure may be used to infiltrate theTRISO surrogate/graphite matrix into the porous support 12.

FIGS. 9A-9D are images of side and top views of another non-limitingembodiment of the nuclear fuel element 10 including the nuclear fuelmaterial 22. In these and other embodiments, surrogate TRISO particlesmay be first infiltrated into the pores 16 between the ligaments 14(“hollow foam ligaments”) thereby leaving gaps between the surrogateTRISO particles and the ligaments 14. Next, the gaps between theparticles and the ligaments 14 may be infiltrated with a carbon-bearingresin, followed by pyrolysis. The process of resin infiltration andpyrolysis may be repeated multiple times, which may be similar to themanufacturing of carbon/carbon composites. In certain embodiments, theouter diameter face 30E is substantially free of the voids 20 (see FIGS.9A and 9B) while the voids 20 are present on the first and second faces30A, 30B (see FIGS. 9C and 9D). The phrase “substantially free” asutilized herein with reference to the outer diameter face 30E means thatthe outer diameter face 30E includes the voids 20 in an amount of nogreater than 5, no greater than 1, no greater than 0.1, or no greaterthan 0.01%, based on a total surface area of the outer diameter face30E. In other words, the hollow ligament passages are closed on theouter diameter of the fuel element and open on the ends to allow foraxial flow. In various embodiments, depending on the infiltrationapproach and the size of the hollow ligament passages, a fuel volumeranging from 30% to 45% can be achieved.

FIGS. 10 and 11 are tomography scans illustrating non-limitingembodiments of the nuclear fuel element 10 including the nuclear fuelmaterial 22. In particular, FIG. 10 shows the nuclear fuel element 10including TRISO surrogate/graphite and FIG. 11 shows the nuclear fuelelement 10 including TRISO surrogate/carbon from pyrolyzed resin. Inthese embodiments, the nuclear fuel element 10 are 20 mm diameter×50 mmlong and include the TRISO surrogate/carbon mixture. However, it is tobe appreciated that the dimensions or configuration are not limited forthe nuclear fuel element 10. The still images of the X-ray tomographyscanning show round pockets of densely packed particles (with a brightappearance) within the pores 16 of the nuclear fuel element 10 (whichappear gray) confirming that the pores 16 are filled with the TRISOsurrogate/carbon mixtures and that the TRISO surrogate particles aredensely packed within the pores 16.

in these and other embodiments, the porous support 12 may be infiltratedwith the desired nuclear fuel to the desired overall density by usingchemical vapor infiltration (CVI), or some other vapor, liquid, orphysical deposition process. Typical infiltration levels, depending onthe application, fall in the 10-30 vol % range (added to the 3 vol %dense RVC skeleton). At this stage, the thermal and mechanicalproperties of the foam may be dictated by the infiltrated material. Theoriginal RVC foam skeleton may have little influence on the final foamproperties, and can often be removed through reaction with hydrogen oroxygen, depending on the particular material that was infiltrated.

Chemical vapor infiltration (CVI), a variation of the chemical vapordeposition (CVD) process, may be used primarily for depositing materialinside of the porous foam, felt, mesh, or fibrous preform. The vapordeposition process is an extremely versatile and relatively inexpensivemethod of molecular-forming materials that are difficult to machine orotherwise produce by conventional processes. CVI relies on thedecomposition of a gaseous precursor, flowed over (in the case of CVD),or through (in the case of CVI) a heated substrate, with subsequentcondensation from the vapor state to form a solid deposit on thesubstrate. Benefits of CVD/CVI include the ability to produce depositsof controlled density, thickness, orientation, and composition. Impuritylevels are typically less than 0.1%, with densities up to 99.9% beingachievable. in addition, CVD/CVI coating processes exhibit the greatestthrowing power, or ability to uniformly deposit on intricately shaped ortextured substrates.

Perhaps the greatest benefit of CVD/CVI is that a wide variety ofmaterials can be deposited at temperatures that are 10% to 50% of themelting point of the coating material itself, which eliminates the needto perform liquid-phase infiltration at high temperatures. Inpreparation for infiltration, the RVC foam substrate/skeleton can easilybe machined to near final dimensions, while accounting for minordimensional changes that occur during infiltration.

In the CVI process, reactant gases (typically metal chlorides orfluorides containing the desired coating material(s)) are flowed througha heated substrate (e.g., RVC foam). The compound(s) within the reactantgas stream react near the heated ligament surfaces to form a continuous,uniform coating. For example, NbC is deposited at 1000-1200° C. viareaction of niobium pentachloride (NbCl5) with methane (CH4) andhydrogen (H2).

Coatings of ZrC, TaC or UC can be deposited by analogous reactions. Theprimary process variables that may be optimized are temperature,pressure, reactant concentration and flow rate, and deposition time.Using CVI, multiple materials may be deposited simultaneously in awell-mixed state as a single deposit. Optionally, after CVIinfiltration, exposure to high temperature hydrogen may be used toremove the underlying RVC foam skeleton, and any free carbon in thedeposited coating. Removal of the underlying skeleton using hydrogen oroxygen has virtually no impact on the mechanical performance of thefoam, since the properties are primarily determined by the stiff metalcarbide coating.

In the chemical vapor infiltration (CVI) process for a single metalcarbide, the appropriate metal in pellet form is first chlorinated andthen flowed over a heated substrate. Hydrogen and a carbon source areadded to the system. Through a combination of thermal decomposition andchemical reaction, the carbide deposits on the heated substrate surface,while HCl gas is removed from the reaction chamber by a vacuum system.Deposition of more than one metal carbide simultaneously is morecomplicated because the metal chlorides must be well mixed and in thedesired ratio in order to form a coating of the desired composition andhomogeneity. For the case of the simultaneous deposition of UC, NbC, andZrC, one approach is to chlorinate each metal separately and then mixthe gases together prior to reaching the heated substrate. This approachrequires independent control of three separate chlorine sources touniformly mix the three chlorides.

An alternate approach is to fabricate a pellet containing all of thethree metals mechanically mixed together. In this case, fine powders(e.g., −325 mesh powders, 0.0017″ diameter) are mechanically mixed in anappropriate weight ratio, e.g. 10% U:45% Zr:45% Nb and then mechanicallypressed under high pressure to create a pellet, e.g., a cylindricalpellet 0.5″ dia.×0.25″ long. The “mechanically alloyed” pressed pelletcontaining the three metals is then used in the CVI process describedabove.

Another approach is to manufacture a homogenous pellet that is a truemetallurgical alloy of the three metals. This can be done by, forexample, by liquid-phase sintering at very high temperatures. A eutecticalloy of the two or three-carbide alloys can be produced this way.

In general, fine-grained, fully dense coatings deposited by CVD havebetter stiffness and strength than do bulk materials having the samecomposition fabricated by powder processing. The elastic moduli of suchCVD films have been regularly measured up to 25% higher than those ofthe bulk materials. RVC foam is extremely well-suited as a lightweightsubstrate onto which very high-stiffness coatings may bedeposited/infiltrated by CVD/CVI. Since the modulus of the depositedfilm is so much greater than that of the vitreous carbon foam skeleton,the carbon foam has essentially no influence on the properties of thefinal product; it merely acts as a “locator” for the deposited films.Ceramic foams fabricated via CVI exhibit significantly greater thermaland mechanical fracture toughness than do monolithic ceramics since theligamental structure severely inhibits crack propagation.

Optionally, a protective coating, e.g. ZrC, may be vapor deposited ontop of the layer(s) of nuclear fuel as an additional moderator orencapsulation barrier. The protective coating can contributesignificantly to the overall porous body's strength

Other coating techniques may be used to deposit the nuclear fuel andrefractory metal carbides or carbonitrides, including chemical reactiondeposition (CRD), physical vapor deposition (PVD), electrolyticdeposition (ED), cathophoresis deposition (CD), electrophoresisdeposition (ED), and sol-gel coating (SGC), and a liquid “painting”technique that uses vacuum infiltration to draw a suspension of finepowder in a liquid. binder into the porous body, followed by baking todrive off the liquid binder. Also, a “melt-infiltration” process may beused as a method of introducing the desired metals into the foamstructure and coating the ligaments, followed by conversion to atricarbide form. Also, the fuel material may be “cast” into athermally/structurally stable foam material.

The above description is that of current embodiments of the invention.Various alterations and changes can be made without departing from thespirit and broader aspects of the invention as defined in the appendedclaims, which are to be interpreted in accordance with the principles ofpatent law including the doctrine of equivalents. Any reference toelements in the singular, for example, using the articles “a,” “an,”“the,” or “said,” is not o be construed as limiting the element to thesingular.

1. A nuclear fuel element, comprising: a porous support comprising aligament and defining a pore adjacent to the ligament, the ligamenthaving an interior surface spaced from the pore, the interior surfacedefining a void within the ligament, the porous support comprisingsilicon carbide; and a nuclear fuel material disposed in the pore, thenuclear fuel material comprising a moderator and tri-structuralisotropic (TRISO) particles.
 3. The nuclear fuel element of claim 1,wherein the void is substantially free of the nuclear fuel material. 4.The nuclear fuel element of claim 1, wherein the pore has a volume, thepore includes the nuclear material in an amount of at least 50 vol %based on the volume of the pore.
 4. The nuclear fuel element of claim 1,wherein the ligament is further defined as a hollow ligament definingthe void extending from a first opening to a second opening.
 5. Thenuclear fuel element of claim 1, wherein porous support comprises aplurality of the ligaments with the voids of the ligaments in fluidcommunication with each other.
 6. The nuclear fuel element of claim 1,wherein the moderator comprises graphite.
 7. The nuclear fuel element ofclaim 6, wherein the nuclear fuel material comprises a slurry of TRISOparticles and graphite.
 8. The nuclear fuel element of claim 1, whereinthe nuclear fuel material further comprises zirconia.
 9. A nuclear fuelelement, comprising; a porous support comprising a ligament and defininga pore adjacent to the ligament, the ligament having an interior surfacespaced from the pore, the interior surface defining a void within theligament, the ligament comprising a nuclear fuel material; and facesheetoverlying the porous support and defining a hole, the hole in fluidcommunication with the void; wherein the nuclear fuel material comprisesa nuclear fuel.
 10. The nuclear fuel element of claim 9, wherein thevoid is substantially free of the nuclear fuel.
 11. The nuclear fuelelement of claim 9, wherein the pore is substantially free of thenuclear fuel.
 12. The nuclear fuel element of claim 9, wherein theligament is formed from the nuclear fuel.
 13. The nuclear fuel elementof claim 9, wherein the porous support comprises niobium.
 14. Thenuclear fuel element of claim 9, wherein the porous support comprises aplurality of the ligaments with the of the ligaments in fluidcommunication with each other.
 15. The nuclear fuel element of claim 14,wherein the facesheet defines a plurality of the holes, the holes influid communication with the voids.
 16. The nuclear fuel element ofclaim 15, wherein the porous support has a first face, a second faceopposite the first face, a third face transverse to the first face, anda fourth face opposite the third face.
 17. The nuclear fuel element ofclaim 16, wherein a first facesheet overlies the first face and a secondfacesheet overlies the second face and wherein the ligaments adjacentthe third. face and the fourth face are substantially free of the voids.18. The nuclear fuel element of claim 9, wherein the facesheet comprisesniobium.
 19. The nuclear fuel element of claim 9, wherein the nuclearfuel comprises uranium dioxide (UO₂) and uranium acetylide (UC₂). 20.The nuclear fuel element of claim 9, wherein the nuclear fuel materialfurther comprises a cladding overlying the nuclear fuel.